Footwear such as disposable slippers issued, for example, to patients in hospitals and medical clinics, and insoles for slippers and shoes, have been made from a variety of paperlike and rubberlike materials. These have a number of disadvantages.
Such footwear made from paperlike materials lack durability and wet strength. Rubberlike materials have other disadvantages as described below.
Separable insoles are conventionally made of soft, resilient rubber, either sponge rubber, or soft rubber containing discrete holes or pockets. Some are laminated with additional materials to provide strength but rubber or a rubberlike synthetic is a basic, resilient, elastic component. Examples are shown in Cooney U.S. Pat. No. 1,596,923; Hitzler U.S. Pat. No. 1,869,257; Kelly U.S. Pat. No. 2,007,803; and Wilson U.S. Pat. No. 2,090,881.
Rubber is not very satisfactory for this use, either in disposable slippers, or insoles, because it is by nature basically resilient and elastic and for foot comfort it has been considered necessary to use it in soft grades. In soft grades and thin sections rubber has no useful tensile strength, so it flattens down readily under the pressure of a wearer's foot. If such soft rubber is used for the sole of a disposable slipper, or a separable insole for a permanent shoe, it seals the bottom of the foot and permits very limited ventilation only to the extent that air can pass through the pores of the material. Such prior soles and insoles have no effective provision for the movement of ventilating air freely back and forth across the foot surface. If the rubber has holes, they are closed by the overlying foot, and if the rubber has air pockets, they are flattened. As a result, and despite advertising claims to the contrary, conventional slipper soles and insoles made of rubber or rubberlike resilient materials are not adequate or effective to cushion, ventilate, and cool the feet.
Further, these conventional soles and insoles are expensive. It is not uncommon for ordinary non-name-brand insoles to cost $3.00 or more per pair. The cost creates an incentive for the wearer to keep them far beyond their useful life, causing the feet to heat and perspire under a sort of "plastic raincoat effect" as they cling to the bottom of the feet and restrict air circulation.